The embodiments described herein relate to controlling operation of power generation and delivery systems, and more specifically, to stabilizing a power converter after an electrical grid contingency event.
Wind turbine generators utilize wind energy to produce electrical power. Wind turbine generators typically include a rotor having multiple blades that transform wind energy into rotational motion of a drive shaft, which in turn is utilized to drive an electrical generator to produce electrical power. Each of the multiple blades may be pitched to increase or decrease the rotational speed of the rotor. A power output of a wind turbine generator increases with wind speed until the wind speed reaches a rated wind speed for the turbine. At and above the rated wind speed, the wind turbine generator operates at a rated power.
Variable speed operation of the wind turbine generator facilitates enhanced capture of energy by the wind turbine generator when compared to a constant speed operation of the wind turbine generator. However, variable speed operation of the wind turbine generator produces electricity having varying voltage and/or frequency. More specifically, the frequency of the electricity generated by the variable speed wind turbine generator is proportional to the speed of rotation of the rotor. A power converter may be coupled between the electric generator and an electrical grid. The power converter outputs electricity having a fixed voltage and frequency for delivery on the electrical grid.
Power generated by an electric utility, using renewable sources of energy or fossil fuel based sources of energy, is typically delivered to a customer over an electrical grid. Electricity applied to the electrical grid is required to meet grid connectivity expectations. These requirements address safety issues as well as power quality concerns. For example, the grid connectivity expectations include operating the power generation system during a transient event, also referred to herein as a grid contingency event. Transient events may include grid fault conditions and weak grid conditions. This capability may be referred to as low voltage ride through (LVRT) or zero voltage ride through (ZVRT) event. An LVRT/ZVRT event is a condition where the alternating current (AC) utility voltage is low on either one phase of the electrical grid or multiple phases of the electrical grid
During an LVRT/ZVRT event, the capacity of the electrical grid to accept power from the power generation system is low. Following switching actions in the external grid, the impedance of the grid may increase substantially leading to a condition referred to herein as a “weak grid”. A weak grid may also arise in situations where a power generation system is connected to a grid in a location remote from other generation sources. In this type of situation, the ability to maintain grid connectivity through grid events may be reduced because a weaker alternating current transmission is available for the power generation system to synchronize its phase. When wind turbines are located in a weak grid, wind turbine power fluctuations may lead to an increase in magnitude and frequency variations in the grid voltage as measured by the wind turbine generator. These fluctuations may adversely affect the performance and stability of the wind turbine phase control.
Operation of the power converter is controlled to facilitate handling of the occurrence of grid contingency events. Once the grid contingency event dissipates, the power converter is controlled to facilitate recovery from the event and return the power generation system to steady-state operation. During the recovery, system oscillations may cause instability, for example, instability in a power output by the power converter.